Images
Formula: Fe3+2O(SO4)2.8H2O
Hydrated sulphate
Crystal System: Triclinic
Specific gravity: 2.255 measured, 2.250 calculated
Hardness: 3
Streak: Orange-yellow
Colour: Brown to burnt orange, light amaranth-red
Luminescence: Not fluorescent under UV
Solubility: Decomposed by hot water but virtually insoluble in cold water, readily soluble in
hydrochloric acid
Environments
Hohmannite is a low-temperature precipitate in oxidised iron sulphide
deposits
(Webmin); it rapidly dehydrates to metahohmannite
on exposure to air
(AM 23.746).
Localities
At the type locality, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, hohmannite is
always intimately associated with chalcanthite and
picromerite. The
chalcanthite is an earlier mineral and is believed to
be formed from an earlier iron-chalcanthite
(copper-bearing
melanterite?). Hohmannite is later than both
fibroferrite and
copiapite and earlier than
amarantite
(AM 23.746). Sideronatrite is another
associated mineral
(Mindat).
At the Redington mine, Knoxville, Knoxville Mining District, Napa county, California, USA,
hohmannite occurs in opaline.
Opal is the principal constituent with some
secondary
chalcedony, disseminated
pyrite and marcasite,
possibly millerite, and relict grains of
picotite from the
peridotite stage.
All specimens contain minute well-formed sulphur crystals
implanted on the hohmannite and a few of the specimens show
cinnabar that occurs as a coating on the
opaline fragments and seems to be earlier than the
hohmannite. The following sulphate minerals have been found at this mine:
copiapite,
coquimbite,
botryogen,
redingtonite and
knoxvillite.
The hohmannite consists of a massive aggregate of minute subhedral crystals out of which have
grown clusters and single prismatic crystals. The massive hohmannite is burnt-orange in colour
and the crystals are dark brown.
Hohmannite is unstable above 27"C. Its ultimate source is probably the
marcasite and pyrite
that are prominent in some parts of the mine
(AM 16.396-404 as castanite).
Back to Minerals